In the late 1990s, new services including online surfing, video multicast, Video on Demand (VoD), Voice over Internet Protocol (VoIP), etc, emerged one after another along with the rapid development of Internet, and the new services have imposed higher requirements on network bandwidth. Varieties of access network technologies have been developed to satisfy the demand of users for bandwidth, e.g., Integrated Services Digital Network (ISDN), coaxial cable modem, Asymmetrical Digital Subscriber Loop (ADSL), Very High Speed Digital Subscriber Loop (VDSL), etc.
Among the emerging new access network technologies, ADSL provides a high-speed data transmission on twisted pairs based on the existing narrow band telephony network, which is favorable for conventional operator to support new services with a low cost and a high efficiency, hence the ADSL technology is very promising. Though ADSL technology appeared more than 10 years ago, it is still the mainstream broadband access technology and will probably remain popular for another couple of years.
Being challenged by other new technologies, ADSL technology does not stop evolving but keeps on developing and perfecting the technology itself. The authority that issues the ADSL standards, since the issuance of the first ADSL standard G.992.1 in 1999, has issued ADSL2 standard G.992.3 and ADSL2+ standards G.992.5. The development of ADSL technology is directly reflected most in the ever increasing transmission bandwidth: at first the download transmission rate supported by ADSL technology was 6144 k and the upload transmission rate was 896 k, now ADSL2+ supports an upload transmission rate of millions of bits and a download transmission rate of tens of million bits.
Another mainstream broadband access technology other than ADSL technology is VDSL technology. VDSL technology also provides broadband services based on twisted pairs of narrowband telephony network. The frequency spectrum of VDSL technology is wider. Therefore the data transmission rate provided by VDSL technology can be as high as 200 Mb. However, while providing services, VDSL technology requires a shorter distance between a central office and end users and consequently certain modifications of the existing networks. Therefore VDSL technology has not been applied in large scale. Greater development is expected in a few years.
A conventional method for carrying multiple services at an XDSL physical port of a single latency path achieves multiple services recognition only, i.e., on either single Virtual Channel (VC) or multiple VCs, the services are transmitted on the same latency path, thus the transmission latency and quality of different services are also the same, which makes it impossible to fulfill different requirements of different services on transmission latency and transmission quality, e.g., VoIP service requires short latency and tolerates low transmission quality to some extent while video multicast or VoD service is very sensitive to frame loss in the transmission. When the transmission latency at the XDSL port is short, the requirements of voice services on time latency will be satisfied while the requirements of video services on frame loss rate can not be satisfied. When the transmission latency at the XDSL port is long, the requirements of video services on frame loss rate will be satisfied while the requirements of voice services on time latency can not be satisfied. As a consequence, when the XDSL port supports a single latency path, the requirements of varieties of services are all taken into consideration while setting the maximum transmission latency at the XDSL port, which results in a poor quality in general for all services and eventually low satisfaction of users.